Optical storage media are media in which data are stored in an optically readable manner, for example by means of a pickup comprising a laser for illuminating the optical storage medium and a photo-detector for detecting the reflected light of the laser beam when reading the data. In the meanwhile a large variety of optical storage media are available, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to about 50 Gigabyte (GB). Digital data are stored in these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-ray Disc, which allows to store up to 50 GB on a dual layer disc. Available formats are at present for example read-only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-ray Disc, a pickup with a laser wavelength of 405 nm is used. On the Blu-ray Disc a track pitch of 320 nm and a mark length from 2T to 8T, maximum 9T, is used, where T is the channel bit length, which corresponds with a minimum mark length of 138-160 nm.
The diffraction limit of optical instruments as described by the Abbe theory is about lambda/2NA, which is 238 nm for a Blu-ray type pickup with a wavelength lambda=405 nm and a numerical aperture NA=0.85. This theoretical minimal detectable length from the diffraction theory is corresponding to a period of the pattern function, which is formed of a pit and of a land having the same length. The smallest detectable element of such a system is a pit or a land having a length of about lambda/4NA, which corresponds for a Blu-ray type pickup with a length of 119 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-ray Disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. For example, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and which center reflectivity is dependent on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the optical resolution limit of lambda/4NA of a corresponding optical pickup.
The nonlinear layer is often called a super-resolution near-field structure (Super-RENS) layer because it is assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. Super-RENS optical discs comprising a super-resolution near-field structure formed of a metal oxide, a polymer compound or a phase-change layer comprising GeSbTe or AgInSbTe are known.
WO 2006/004338 discloses an apparatus comprising a pickup for reading data from a super-resolution optical disc, wherein the pickup provides a first beam having a light intensity being sufficient for providing a super-resolution effect and a second beam following the first beam having not the light intensity for providing the super-resolution effect. By taking into account a temporal delay between the reflected signal of the first beam and the reflected signal of the second beam, reflected light outside of a reproduction beam spot of the super-resolution area is excluded, thereby improving the reproduction signal characteristics of the HF-signal.
The publication “Phase transformation of an InSb surface induced by strong femtosecond laser pulses” by Shumay and Höfer, Physical Review B, Vol. 53, No. 23, 15 Jun. 1996, p. 15878-16884, describes how phase transformations can be triggered on an InSb surface by applying high power femtosecond laser pulses.